Biodegradable dispersants for cement compositions and methods of cementing in subterranean formations

ABSTRACT

Methods of cementing in a subterranean formation may include the steps of: providing a cement composition comprising a cement, water, and a dispersant comprising a low molecular weight starch that comprises anionic groups; placing the cement composition in the subterranean formation; and allowing the cement composition to set therein. In certain embodiments, improved biodegradable dispersants may include low molecular weight starches having anionic groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to subterranean cementing operations, andmore particularly, to improved cement dispersants for cementcompositions and methods of cementing in subterranean formations.

2. Description of the Prior Art

Hydraulic cement compositions are commonly utilized in subterraneanapplications including but not limited to well completion and remedialoperations. For example, in well applications, hydraulic cementcompositions are used in primary cementing operations whereby strings ofpipe such as casing and liners are cemented in well bores. In performingprimary cementing, a hydraulic cement composition is pumped into theannular space between the walls of a well bore and the exterior surfaceof the pipe string disposed therein. The cement composition is permittedto set in the annular space, thereby forming an annular sheath ofhardened substantially impermeable cement therein that substantiallysupports and/or positions the pipe string in the well bore and bonds theexterior surfaces of the pipe string to the walls of the well bore.Hydraulic cement compositions are also used in remedial cementingoperations such as plugging highly permeable zones or fractures in wellbores, plugging cracks in holes in pipe strings, and the like.

Dispersants are often used in subterranean well cement compositions tofacilitate mixing the cement composition. Such dispersants areextensively used, inter alia, to reduce the apparent viscosities of thecement compositions in which they are utilized to allow the cementcomposition to be pumped with less friction pressure and lesshorsepower. In addition, the lower viscosity often allows the cementcomposition to be pumped in turbulent flow. Turbulent flowcharacteristics are desirable, for instance, when pumping cementcompositions into subterranean wells to more efficiently remove drillingfluid from surfaces in the well bore as the drilling fluid is displacedby the cement composition being pumped. The inclusion of dispersants incement compositions is also desirable in that the presence of thedispersants may facilitate the mixing of the cement compositions andreduce the water required. This may be desirable because cementcompositions having reduced water content are often characterized byimproved compressive strength development.

A number of dispersing agents have been utilized heretofore in cementcompositions, particularly in cement compositions used for primary andremedial cementing in subterranean wells. One of the most common cementslurry dispersants is a condensate product of sulfonated naphthalene andformaldehyde. Such dispersants are problematic, however, because theyare not substantially biodegradable, and hence, do not meet theregulatory requirements in some countries for use as dispersants.

Another conventional cement composition dispersant is the condensationproduct of formaldehyde, acetone, and an alkali metal sulfite. Oneformulation of this conventional dispersant is commercially availableunder the trade designation “CFR-3” from Halliburton Energy Services,Inc., of Duncan, Okla. While this and other similar dispersants mayfunction well as dispersants in cement compositions, they are oftenenvironmentally unacceptable for use in wells subject to more stringentenvironmental regulations. Their unacceptability in these environmentsstems from, inter alia, their inability to undergo completebiodegradation in the environment, which may result in undesirableenvironmental effects if either accidentally or intentionally releasedinto the environment.

SUMMARY OF THE INVENTION

The present invention provides biodegradable cement slurry dispersants,cement compositions comprising such dispersants, and methods ofcementing in a subterranean formation.

In certain embodiments, the biodegradable dispersants of the presentinvention comprise a low molecular weight starch that comprises anionicgroups.

In one embodiment, the methods of cementing in a subterranean formationof the present invention comprise providing a cement compositioncomprising a hydraulic cement, water, and a biodegradable dispersant ofthe present invention; placing the cement composition in a subterraneanformation; and allowing the cement composition to set therein.

The objects, features and advantages of the present invention will bereadily apparent to those skilled in the art upon a reading of thedescription of the preferred embodiments that follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The biodegradable dispersants of the present invention comprise astarch-based material. Starch is a polymer of glucose in whichmonosaccharide units are linked by 1,4′-α-glycoside bonds. Starchessuitable for use in accordance with the present invention may beobtained from renewable glucose sources such as potatoes, corn, maize,tapioca, other cereal grains, and the like. Combinations of starchesfrom combinations of sources are also suitable. The starch-baseddispersants of the present invention are advantageous, because they are,inter alia, biodegradable and do not cause undesirable effects on theenvironment.

The starches of the present invention preferably should be characterizedby a low molecular weight, in the range of about 500 to about 10,000, ormore preferably about 1,000 to about 5,000. Examples of low molecularweight starches include dextrinized starches and starches degraded withacids, peroxides, or other oxidizing agents. For dextrinized starches,heat or acid treatment methods may reduce the molecular weight of thestarch. These starches can be used with or without reacting them withsuitable reagent groups to add anionic groups that may increase theirdispersing ability as discussed below.

In the case of non-oxidized, low molecular weight starches such as acid,peroxide, or heat degraded starches, the starch can be further reactedwith suitable reagents to introduce anionic groups. Similarly, toenhance the dispersing action of any suitable starch, the starch may bemodified to incorporate anionic charges. Anionic groups that may beuseful include but are not limited to: sulfite, sulfate, sulfonate,carboxylate, silicate, nitrate, and nitrite groups. These anionic groupscan be added to the starch by reacting the starch with an agent capableof producing the anionic groups, including but not limited tohypochlorite, peracetic acid, hydrogen peroxide, sodium periodate,sodium persulfate, sodium percarbonate, propane sultone, butane sultone,chloroacetic acid, dinitrogen tetroxide, 3-chloropropyl-triethoxysilane, 3-glycidoxy-propyltrimethoxy-silane, or the like.

Additionally, when the starch contains substantially carbonyl groups,particularly aldehyde groups, it can be further reacted with a sulfitesalt to provide a sulfite adduct of an oxidized starch; sulfite adductsof oxidized starches are especially suitable for use in accordance withthe present invention. Another method of forming a sulfite adduct of anoxidized starch includes reacting an acetone formaldehyde condensatewith starch under alkaline conditions, followed by addition of a sulfitesalt as described in U.S. Pat. No. 5,247,086, incorporated herein byreference. Other suitable starches include propylene oxide modifiedstarches, ethylene oxide modified starches, and lightly crosslinkedstarches.

In preferred embodiments of the present invention, the starch in thedispersant is oxidized. Oxidation of starches either with hypochlorite,hydrogen peroxide, sodium periodate, or the like is suitable as suchoxidation is likely to produce starch molecules that have carbonylgroups, particularly aldehydes, and carboxylate groups. When theoxidized starch comprises carboxylate groups, the material itself may beused as dispersant or further reacted with a sulfite salt to producedispersant containing both sulfite and carboxylate anionic groups.

The anionic groups discussed above are thought to provide, inter alia,dispersing action in highly viscous slurries, e.g., slurries having alow water to cement ratio, or slurries containing a large fraction ofsolids, without substantially affecting the time in which the cementcomposition sets therein.

The solubility and storage characteristics of the dispersants of thepresent invention depend at least in part on the structural features ofthe starch or starches utilized. For example, one type of starchcontains two components, namely a linear polymer molecule referred to asamylose, and a branched polymer molecule referred to as amylopectin.High amylose starches are typically not cold-water soluble. Heating suchstarch suspensions usually dissolves the starch. Upon cooling, thesolutions become viscous gels. Additionally, these solutions maycontinue to viscosify with time due to the alignment of linear starchmolecules in a process called retrogradation. Such high amylosestarches, when modified to function as dispersants for cement slurries,may be more suitable for dry blending. On the other hand, highamylopectin-containing starches are significantly more cold-watersoluble and the solutions are less likely to gel at room temperature.Such starches may be more amenable to a liquid form. Additionally, aswill be recognized by those skilled in the art with the benefit of thisdisclosure, the water solubility of starches can further be increased byintroducing low levels of cationic groups in the form of quaternaryammonium groups. Such cationizations are carried out by reacting starchwith, e.g., glycidylpropyltrialkylammonium salts. This may decrease thetendency of a starch to gel while stored. Other modified starches usefulin the present invention include propylene oxide modified starches,ethylene oxide modified starches, and cross-linked starches.

The biodegradable dispersants of the present invention may be utilizedin any subterranean cementing applications. All cements suitable for usein subterranean cementing operations may be used in accordance with thepresent invention. In one embodiment, the improved cement compositionsof the present invention comprise a hydraulic cement, sufficient waterto form a pumpable slurry, and a biodegradable starch-based dispersantof the present invention in an amount effective to reduce the apparentviscosity of the cement composition prior to when it gels and sets. Incertain embodiments, the biodegradable dispersant of the presentinvention is included in the cement composition in the amount of fromabout 0.01% to about 5.0%. In other embodiments, the dispersant may beadded in a range of from about 0.2% to 3.0%. A variety of hydrauliccements can be utilized in accordance with the present inventionincluding those comprised of calcium, aluminum, silicon, oxygen, and/orsulfur, which set and harden by reaction with water. Such hydrauliccements include, but are not limited to, Portland cements, pozzolanacements, gypsum cements, high alumina content cements, silica cements,and high alkalinity cements. Portland cements are generally preferred.In some embodiments, the Portland cements that are suited for use inconjunction with the present invention are classified as Class A, C, H,and G cements according to American Petroleum Institute, APISpecification for Materials and Testing for Well Cements, APISpecification 10, Fifth Ed., Jul. 1, 1990. Another useful cement forcertain embodiments of the present invention include a cement that iscommercial available under the tradename “THERMALOCK™” from HalliburtonEnergy Services, Inc., in Duncan, Okla., and described in U.S. Pat. No.6,488,763, incorporated herein by reference. The dispersants of thepresent invention are also suitable for use with low-density cements.Such low-density cements may be foamed cements or may be cementscomprising another means to reduce their density such as hollowmicrospheres, low-density elastic beads, or other density-reducingadditives known in the art.

The dispersants of the present invention can be used in a liquid orsolid form, depending on the application. For example, a dispersant ofthe present invention in a powder form can be blended with dry cementprior to mixing with water. Such dry blends are generally preferred whenthe wells to be cemented are on land. In offshore applications, it ispreferred that the dispersants are in a solution or suspension form.

The water utilized in the cement compositions of this invention can befresh water, salt water (e.g., water containing one or more saltsdissolved therein), brine (e.g., saturated salt water produced fromsubterranean formations), or seawater. Generally, the water can be fromany source provided that it does not contain an excess of compounds thatadversely affect other components in the cement composition. The waterpreferably is present in an amount sufficient to form a pumpable slurry.More particularly, the water is present in the cement compositions in anamount in the range of from about 30% to about 110% by weight ofhydraulic cement therein, more preferably in an amount of about 40%.

One example of a composition of the cement compositions of the presentinvention is comprised of a hydraulic cement; water present in an amountin the range of from about 30 to about 110% by weight of cement in thecomposition; and a starch-based biodegradable dispersant of the presentinvention, present in an amount in the range of from about 0.2 to about3.0% by weight of cement in the composition.

As will be recognized by those skilled in the art, when the cementcompositions of the present invention are utilized for primary orremedial subterranean well operation, such compositions can also includeadditional additives such as fluid loss additives, weighting materials,light weight material, set retarders, accelerators, and the like. Thecement compositions also can include other additives such asaccelerators or retarders, if desired. If an accelerant is used, it ispreferably calcium chloride. Also, if used, in certain embodiments, suchaccelerants are present in an amount in the range from about 1.0% toabout 4.0% by weight of the cement in the compositions. Fluid lossadditives such as hydroxyethylcellulose, carboxymethylcellulose,carboxymethylhydroxyethylcellulose, hydroxypropylcellulose,hydroxypropylguar, guar, polyvinylalcohol, synthetic polyelectrolytesare also suitable.

To facilitate a better understanding of the present invention, thefollowing examples of preferred embodiments are given. In no way shouldsuch examples be read to limit the scope of the invention.

EXAMPLES

A number of test cement compositions were prepared using a StandardGrade Class A hydraulic cement mixed with a sufficient amount of freshwater to form a pumpable slurry. One of the test compositions containeda conventional dispersant, specifically CFR-3 dispersant, and othersamples contained examples of dispersants of the present invention.Another control sample was tested wherein no dispersant was added to thecement composition. In addition, to illustrate the effects of oxidizingthe starch, another control sample was tested wherein the starch in thestarch-based dispersant was not oxidized prior to combining it with thecement.

Tables 1, 2 and 3 list the characteristics of the illustrativedispersants of the present invention that were tested. Table 4 indicateshow much and which type of the dispersant of the present invention wereadded to a given cement composition. Tables 5A and 5B list therheological properties of the cement compositions comprising thedispersants of the present invention. The Comparative Tables belowillustrate the properties of the control samples discussed above.

TABLE 1 Examples of Dispersants of the Present Invention. ReactionReaction Weight % Weight % Weight % Temperature Time Sample StarchStarch Bisulfite Sulfite (° C.) (hours) A Dialdehyde Starch 1.7 1.9 — 5616 B Dialdehyde Starch 3.8 — 5.2 63 8.5 C Dialdehyde Starch 5.3 — 3.6 725

Dispersant No. A was made as follows. A suspension of a dialdehydestarch (1.7% by weight) purchased from Sigma-Aldrich Chemical Company inwater was mixed with sodium bisulfite (1.9% by weight) and reacted at56° C. for 16 hours. The dialdehyde starch is an oxidized starch. Theconcentration of solids in the final mixture was 8.39%; the solutiondensity was 1.05 g/cc. The reaction mixture was a colorless solution.1.0% by weight of this dispersant was added to a standard cement mixtureto form Cement Composition No. 2 shown in Table 4.

Dispersant No. B was made as follows. A suspension of a dialdehydestarch (3.8% by weight) from Sigma-Aldrich Chemical Company was mixedwith 5.2% by weight sodium sulfite and 225.4 grams of water at 63° C.for 8.5 hours to obtain a dark brown colored suspension. The resultantsuspension had a density of 1.2 g/cc and a solids concentration of 10%.1.0% by weight of this dispersant was added to a standard cement mixtureto form Cement Composition No. 3 shown in Table 4.

Dispersant No. C was made as follows. A suspension of a dialdehydestarch (5.3% by weight) from Sigma-Aldrich Chemical Company was mixedwith 3.6% by weight sodium sulfite and 225.4 grams of water. Thecomponents were reacted at 72° C. for 5 hours to obtain a dark browncolored suspension. The resultant suspension had a density of 1.2 g/ccand a solids concentration of 10%. 0.2% by weight of this dispersant wasadded to a standard cement mixture to form Cement Composition No. 7shown in Table 4.

TABLE 2 Examples of Dispersants of the Present Invention. Weight %Weight % Weight % Reaction Reaction Sample Starch Starch HypochloriteSulfite Temp. Time D High Amylose 8.2 0.8 4.7 53 3 Starch¹ E HighAmylose 8.1 0.8 4.6 53 24 Starch¹ ¹Obtained from National Starch andChemical Company of Bridgewater, New Jersey.

Dispersant No. D was made as follows. A suspension of a high amylosestarch was obtained from National Starch. 8.2% by weight of the starchwas mixed with 0.8% of hypochlorite to oxidize the starch. The oxidizedstarch was reacted with 4.7% by weight of sodium sulfite at 53° C. for 3hours to produce the dispersant. The concentration of solids in thedispersant was 11.5% and the density was 1.07 g/cc. The pH was 9.3. 0.2%by weight of this dispersant was added to a standard cement mixture toform Cement Composition No. 9 shown in Table 4.

Dispersant No. E was made as follows. A suspension of a high amylosestarch obtained from National Starch and Chemical Company containing8.1% by weight of the starch was mixed with 0.8% of hypochlorite tooxidize the starch. The oxidized starch was reacted with 4.6% by weightof sodium sulfite at 53° C. for 24 hours to produce the dispersant. Theconcentration of solids in the dispersant was 12.7% and the density was1.06 g/cc. The pH was 7.1. 0.2% by weight of this dispersant was addedto a standard cement mixture to form Cement Composition No. 10 shown inTable 4.

TABLE 3 Examples of Dispersants of the Present Invention. Weight %Weight % Weight % Reaction Reaction Sample Starch Starch PeriodateSulfite Temp. Time F High Amylose Starch² 7.3 0.8 0.5 72 24 G HighAmylose Starch² 7.3 0.8 0.5 72 24 ²Obtained from National Starch andChemical Company of Bridgewater, New Jersey

Dispersant No. F was made as follows. A suspension of a high amylosestarch was obtained from National Starch and Chemical Company containing7.3% by weight of the starch. This was mixed with 0.8% of periodate tooxidize the starch and allowed to react for about 16 hours. The oxidizedstarch was reacted with 0.5% by weight of sodium sulfite at 72° C. for24 hours to produce the dispersant. The concentration of solids in thedispersant was 7.8% and the density was 1.05 g/cc. 0.2% by weight ofthis dispersant was added to a standard cement mixture to form CementComposition No. 11 shown in Table 4.

Dispersant No. G was made as follows. A suspension of a high amylosestarch was obtained from National Starch and Chemical Company containing7.3% by weight of the starch mixed with 0.8% of periodate to oxidize thestarch and 0.5% by weight of sodium sulfite at 72° C. for 24 hours toproduce the dispersant. The concentration of solids in the dispersantwas 7.8% and the density was 1.05 g/cc. 0.2% by weight of thisdispersant was added to a standard cement mixture to form CementComposition No. 12 shown in Table 4.

To examine the effect of these examples of the dispersants of thepresent invention on the rheology of a typical cement slurry, each ofthese dispersants was added to a standard cement mixture. The cementmixture for each sample was a mix of a conventional hydraulic cementcommonly used in subterranean well applications and a sufficient amountof water to form a slurry. A sufficient amount of each dispersant wasadded to the cement/water slurry to form a slurry having about 16.4lbs/gallon density. Table 4 lists the particular dispersant and theamount of which was added to the cement slurry. The particulardispersants are listed above in Tables 1–3.

TABLE 4 Cement Compositions Comprising Examples of Dispersants of thePresent Invention. Weight % of Dispersant Cement Added to CompositionDispersant Cement 2 A 1.0% 3 B 1.0% 4 C 0.2% 5 D 0.2% 7 E 0.2% 8 F 1.0%9 G 0.2% 10 H 0.2% 11 I 0.2% 12 J 0.2%

Tables 5A and 5B list the rheological properties of each of these cementcompositions containing the illustrative dispersants of the presentinvention. The rheological properties of the test cement compositionswere determined in accordance with the Recommended Practice for TestingWell Cements, API Recommended Practice 10B, 22nd Edition, datedDecember, 1997, of the American Petroleum Institute.

TABLE 5A Rheological Properties of Cement Slurries ComprisingDispersants of the Present Invention Shown in Table 4. Cement CementCement Cement Cement Compo- Compo- Compo- Compo- Compo- Viscosity,sition sition sition sition sition (cp) No. 2 No. 3 No. 4 No. 5 No. 7300 rpm 49 33 49.5 43 46 200 rpm 34 22 36 31 33.5 100 rpm 18 11 23 19 20 60 rpm 12 7 17 14 14  30 rpm 7 4 14 10 9.5  6 rpm 3 2 8 6.5 5.5  3 rpm3 1.5 7 6 5 600 rpm 99 73 93 64 93

TABLE 5B Rheological Properties of Cement Slurries ComprisingDispersants of the Present Invention Shown in Table 4. Cement CementCement Cement Cement Compo- Compo- Compo- Compo- Compo- Viscosity,sition sition sition sition sition (cp) No. 8 No. 9 No. 10 No. 11 No. 12300 rpm 39 41 43 43 74 200 rpm 26 28 30 30 55 100 rpm 13.5 15 18 16 36 60 rpm 8.5 11 13 11 28  30 rpm 5 6 9 7 23  6 rpm 1.5 3.5 6 4.5 19  3rpm 1 3 5.5 4 16 600 rpm 85 82 84 not 125 determined

Another example was performed to illustrate, inter alia, that thesulfite adducts of an oxidized starch will disperse cement slurries. A100 g sample of a 10% suspension in water of an oxidized starch, “D17F”obtained from Grain Processing Corp., of Muscatine, Iowa, was heated to185° F. for 6 hours, at which time the suspension became stable tosettling. The pH of the suspension was adjusted to 12.2, and 0.75 g ofsodium sulfite was added. The mixture was heated to 185° F. for 4 hours.The resulting light yellow turbid mixture was tested in the cementslurry for its dispersing activity as follows. For comparison, a sampleof the starch suspension after adjusting the pH but prior to theaddition of sodium sulfite was also tested.

A cement slurry having a density of 16.4 lbs/gal containing 0.53% of thecontrol oxidized starch by weight of the cement and another similarslurry containing the sulfite adduct of an oxidized starch were preparedand tested for rheology. The results are shown below in Table 6.

TABLE 6 Rheological Properties of Cement Slurries Comprising Dispersantsof the Present Invention. Viscosity Control Oxidized Starch SulfiteReacted Oxidized Starch 300 rpm 90 79 200 rpm 70 58 100 rpm 45 33  6 rpm22 10  3 rpm 16 11 600 rpm 150 150

From Table 6, it is evident that the sulfite addition product of anoxidized starch disperses cement slurries better than the controloxidized starch.

To determine how these illustrative dispersants of the present inventioncompared to conventional nonbiodegradable dispersants, a comparativetest was run. To perform these tests, the same standard cementcomposition was made as described above (hydraulic cement and asufficient amount of water to form a slurry) but a conventional widelyaccepted nonbiodegradable dispersant was added. This nonbiodegradabledispersant was CFR-3. 0.2% by weight of the dispersant was added to thecement composition. The resultant cement composition had a density of16.4 lbs/gal.

COMPARATIVE TABLE 1 Control Sample, Cement Composition ContainingConventional Nonbiodegradable Dispersant. Cement % by weight ofComposition No. Dispersant Type dispersant 6 Conventional CFR-3 0.2%

The rheological properties of the cement composition containing theconventional nonbiodegradeable dispersant (Cement Composition No. 6)were determined as reported in Comparative Table 2.

COMPARATIVE TABLE 2 Rheological Characteristics of Cement Composition 6Containing Conventional CFR-3 Composition in Comparative Table 1. RPMViscosity, cp 300 rpm 46 200 rpm 34 100 rpm 20  60 rpm 14.5  30 rpm 10 6 rpm 7  3 rpm 6.5 600 rpm 100

As can be seen from Tables 5A, 5B, and Comparative Table 2, thebiodegradable dispersants of the present invention imparted propertiesto cement compositions which compare closely with the propertiesimparted to the same cement compositions comprising a highly-acceptedconventional nonbiodegradable dispersant. In particular, CementComposition Nos. 5, 7, 9, 10, and 11 particularly appear to impartcomparable properties to the cement compositions as the conventionalnonbiodegradable dispersant.

Another control sample was also tested to determine the effectiveness ofthe illustrative dispersants of the present invention. In this secondcontrol sample, no dispersant was added to the cement mixture. Thecement mixture contained hydraulic cement and water in an amountsufficient to form a slurry, and had a density of 16.4 lbs/gal.Comparative Table 3 lists the characteristics of this cement mixture.

COMPARATIVE TABLE 3 Control Sample, Cement Composition Having NoDispersant. Composition No. Dispersant Type % by weight of dispersant 1None None

After forming this control sample with no dispersant, the rheologicalproperties of the cement were determined. These are reported inComparative Table 4.

COMPARATIVE TABLE 4 Rheological Properties for Control Sample ofComparative Table 3. RPM Viscosity, cp 300 rpm 102 200 rpm 87 100 rpm 68 60 rpm 60  30 rpm 50  6 rpm 24  3 rpm 17 600 rpm 164

As can readily be seen from examining Tables 5A, 5B, and ComparativeTable 4, the dispersants of the present invention proved to be effectivein reducing the viscosity of the cement mixture to a desirable level.

Another control test was performed to determine the effect of oxidizingthe starch prior to reacting it with a sulfite. The control starch inthis test was used to prepare dispersants D and E in Table 2. Reactingthe oxidized starch with a sulfite is thought to provide anionic sulfitegroups to the starch. The anionic sulfite groups are thought tocontribute to the dispersing action of the dispersant in highly viscousslurries, e.g., slurries having a low water to cement ratio, or slurriescontaining a large fraction of solids.

COMPARATIVE TABLE 5 Cement Composition Wherein the Dispersant is aStarch That Was Not Oxidized or Reacted With a Sulfite. % by weight ofComposition No. Dispersant Type dispersant 13 Starch Solution, NationalStarch 0.2% High Amylose Starch (Not Oxidized)

COMPARATIVE TABLE 6 Rheological Properties for Control Sample ofComparative Table 5. RPM Viscosity, cp 300 rpm 81 200 rpm 53 100 rpm 27 60 rpm 18  30 rpm 10  6 rpm 10  3 rpm 11 600 rpm 135

An examination of data for compositions 9 and 10 in Table 5B andComparative Table 6 reveals that oxidizing the starch followed byreaction with a sulfite ion improves the dispersing characteristics ofthe starch-based dispersants of the present invention.

A high amylopectin starch sample derived from waxy maize containingsiloxy anionic groups and cationic quaternary ammonium groups, availablefrom National Starch and Chemical Company under the trade name “COBOND2500” as a 15% starch solution in water was tested in the same cementslurry at a level of 1% by weight of cement. The rheology results areshown in Table 7.

COMPARATIVE TABLE 7 Rheological Properties of Cement Slurry Comprising aDispersant of the Present Invention. RPM Viscosity, cp 300 rpm 100 200rpm 75 100 rpm 46  6 rpm 23  3 rpm 20 600 rpm 170A comparative slurry without Cobond 2500 provided the following rheologyin Comparative Table 7.

COMPARATIVE TABLE 7 Control Slurry Having No Dispersant. RPM Viscosity,cp 300 rpm 115 200 rpm 102 100 rpm 79  6 rpm 25  3 rpm 18 600 rpm 213

The results from using siloxy-substituted starch show that siloxy anionsubstituted starches also disperse cement slurries.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned as well as those that areinherent therein. While numerous changes may be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

1. A method of cementing in a subterranean formation comprising thesteps of: providing a cement composition comprising a cement, water, anda dispersant comprising a low molecular weight starch that comprisesanionic groups; placing the cement composition in the subterraneanformation; and allowing the cement composition to set therein.
 2. Themethod of claim 1 wherein the dispersant comprises a starch derived fromat least one source selected from the group consisting of: potatoes,corn, maize, tapioca, and other cereal grains.
 3. The method of claim 1wherein the starch has a molecular weight of about 1,000 to about 5,000.4. The method of claim 1 wherein the starch comprises at least onestarch selected from the group consisting of: a dextrinized starch and astarch degraded with an oxidizing agent or heat.
 5. The method of claim1 wherein the starch comprises at least one starch selected from thegroup consisting of: a dialdehyde starch, a high amylose starch, and ahigh amylose starch.
 6. The method of claim 1 wherein the starchcomprises a sulfite adduct of an oxidized starch.
 7. The method of claim1 wherein the dispersant is present in the cement composition in anamount sufficient to reduce the apparent viscosity of the cementcomposition prior to when the cement composition sets.
 8. The method ofclaim 7 wherein the dispersant is present in the cement composition inan amount of from about 0.01% to about 5%.
 9. The method of claim 1wherein the cement comprises a hydraulic cement.
 10. The method of claim9 wherein the hydraulic cement comprises at least one material selectedfrom the group consisting of: calcium, aluminum, silicon, oxygen, andsulfur.
 11. The method of claim 1 wherein the cement comprises alow-density cement.
 12. The method of claim 1 wherein the watercomprises at least one type of water selected from the group consistingof: fresh water, salt water, and brine.
 13. The method of claim 1wherein the water component is present in an amount in the range of fromabout 30% to about 110% by weight of the cement in the cementcomposition.
 14. The method of claim 1 wherein the cement comprises ahydraulic cement, the water is present in an amount from about 30% toabout 110% by weight of the cement in the cement composition, and adispersant is present in an amount in the range of from about 0.2% toabout 3.0% by weight of the cement in the cement composition.
 15. Themethod of claim 1 wherein the starch comprises at least one anionicgroup selected from the group consisting of: a sulfite group, a sulfategroup, a sulfonate group, a carboxylate group, a silicate group, anitrate group, and a nitrite group.
 16. The method of claim 1 whereinthe hydraulic cement comprises at least one hydraulic cement selectedfrom the group consisting of: a Class A cement, a Class C cement, aClass H cement, and a Class G cement.
 17. The method of claim 1 whereinthe cement composition further comprises at least one additive selectedfrom the group consisting of: a fluid loss additive, a weightingmaterial, a light weight material, a set retarder, and an accelerator.18. A method of reducing the viscosity of a cement compositioncomprising the steps of: adding a dispersant to the cement compositionwherein the dispersant comprises a low molecular weight starch havinganionic groups that do not substantially affect the length of timerequired for the cement composition to set; and allowing the cementcomposition to set.
 19. The method of claim 18 wherein the dispersantcomprises a starch derived from at least one source selected from thegroup consisting of: potatoes, corn, maize, tapioca, and other cerealgrains.
 20. The method of claim 18 wherein the starch has a molecularweight of about 1,000 to about 5,000.
 21. The method of claim 18 whereinthe dispersant comprises at least one of the following: a dextrinizedstarch or a starch degraded with an oxidizing agent or heat.
 22. Themethod of claim 18 wherein the starch comprises at least one starchselected from the group consisting of: a dialdehyde starch, a highamylose starch, and a high amylopectin starch.
 23. The method of claim18 wherein the starch comprises a sulfite adduct of an oxidized starch.24. The method of claim 18 wherein the dispersant is present in thecement composition in an amount of from about 0.01% to about 5%.
 25. Themethod of claim 18 wherein the dispersant is present in the cementcomposition in an amount sufficient to reduce the apparent viscosity ofthe cement composition prior to when the cement composition sets. 26.The method of claim 18 wherein the starch comprises at least one anionicgroup selected from the group consisting of: a sulfite group, a sulfategroup, a sulfonate group, a carboxylate group, a silicate group, anitrate group, and a nitrite group.
 27. A method of reducing theviscosity of a cement composition comprising the step of adding adispersant to the cement composition wherein the dispersant comprises alow molecular weight starch having anionic groups wherein the lowmolecular weight starch comprises a sulfite adduct of an oxidizedstarch.
 28. The method of claim 27 wherein the dispersant comprises astarch derived from at least one source selected from the groupconsisting of: potatoes, corn, maize, tapioca, and other cereal grains.29. The method of claim 27 wherein the starch has a molecular weight ofabout 1,000 to about 5,000.
 30. The method of claim 27 wherein thedispersant comprises at least one starch selected from the groupconsisting of: a dextrinized starch and a starch degraded with anoxidizing agent or heat.
 31. The method of claim 27 wherein the starchcomprises at least one starch selected from the group consisting of: adialdehyde starch, a high amylose starch, and a high amylopectin starch.32. The method of claim 27 wherein the dispersant is present in thecement composition in an amount of from about 0.01% to about 5%.
 33. Themethod of claim 27 wherein the dispersant is present in the cementcomposition in an amount sufficient to reduce the apparent viscosity ofthe cement composition prior to when the cement composition sets. 34.The method of claim 27 wherein the starch comprises at least one anionicgroup selected from the group consisting of: a sulfite group, a sulfategroup, a sulfonate group, a carboxylate group,.a silicate group, anitrate group, and a nitrite group.